1. Field of the Invention
The present invention relates to a glass preform material used in precision mold pressing and an optical glass suitable for precision mold pressing, which have low dispersion and high refractive index and low transition point (Tg).
2. Description of Related Art
In late years, in optical instruments remarkably getting smaller and lighter, an aspherical lens is often applied in order to decrease the number of lenses constituting an optical system of an optical instrument. A method in which a lens is produced by press forming a heat-softened glass preform material with a metal molding die having precise molding surface, or a precision mold pressing, is a mainstream of manufacturing a glass aspherical lens.
Since the glass molding obtained by precision mold pressing is available as final products without or almost without grinding and polishing after forming, a lens and the like can be manufactured in high productivity with precision mold pressing. Precision mold pressing method is a suitable method for manufacturing a glass aspherical lens because it is difficult to manufacture an aspherical lens in large amount at a low cost in a former method including grinding and polishing. Furthermore, a precision mold pressing method has high productivity. As a result, spherical lenses and other optical elements of various shapes as well as an aspherical lens are manufactured in a precision mold pressing.
There are two major methods of manufacturing the glass preform material for precision glass forming.
One of these methods is: as disclosed in Japanese Patent Application Publication (Unexamined) Tokukaihei-6-122526 and Tokukaihei-8-319124, flowing molten glass from a discharge pipe, cutting the flowed glass while the flowed glass keeps on a molding die, and successively forming glass perform material; or, as disclosed in Japanese Patent Application Publication (Unexamined) Tokukaihei-7-165431, dropping molten glass from a flow-controlled pipe, keeping the droplets on lower molding die, and forming a glass preform material by pressing the kept glass lump with upper molding die. These methods have high productivity of a glass preform material itself because a glass preform material is directly obtained by hot forming of molten glass in both methods.
The other method is one obtaining a glass preform material by cutting a glass block material. There is a problem in this method that many processes are required from cutting a glass block material to shaping a glass close to a final product. However, there is an advantage that shape variation in precision mold pressing can be reduced on a occasion of obtaining a final product of various shapes such as a lens by precision mold pressing since the glass preform material can be shaped close to a final product such as a lens.
In obtaining a glass molding with precision mold pressing, a heat-softened glass preform material is required to be press formed under high temperature condition in order to print precise molding surface of metal molding die to the glass preform material. Therefore, the applied metal molding die is also exposed to high temperature, and furthermore, to high pressure. As a result, when a glass preform material is heat-softened and press-formed, a precise molding surface of metal molding die often cannot be maintained because of oxidation and erosion of the molding surface and a damage of a mold release membrane provided on the molding surface. Further, the metal molding die itself is damaged easily. Under these conditions, since the metal molding die is required to exchange frequently, the frequency of exchanging the metal molding die increases as a result, the manufacturing in large amount at low cost cannot to be achieved. The transition temperature (Tg) of a glass for a glass preform material used in precision mold pressing is desirably as low as possible from the viewpoints of reducing the above-described damage, maintaining precise molding surface of a metal molding die for a long period and permitting precision mold pressing with low pressure. At present, since a glass of the glass preform material for precision mold pressing having transition point (Tg) of more than 650° C. is difficult to be applied to precision mold pressing, a high refractive index and low dispersion glass having transition temperature (Tg) of not more than 650° C. is required, preferably the transition temperature (Tg) of not more than 640° C., more preferably the transition temperature (Tg) of not more than 630° C. Further, since a devitrification of the glass preform material does not disappear by precision mold pressing, and since a devitrified glass molding cannot be used as an optical element such as a lens, the glass of a glass preform material for precision mold pressing is required to have superior resistance to devitrification property.
As for the property of an optical glass used for an aspherical lens, various optical constants (refractive index (nd) and Abbe number (vd)) are required. Among them, the glasses having high refractive index and low dispersion are required in recent years. Particularly, for an optical design, high refractive index and low dispersion glass is strongly required, wherein a refractive index (nd) and an Abbe number (vd) are within an area surrounded by straight lines that are drawn by connecting point A (nd=1.835, vd=46.5), point B (nd=1.90, vd=40.0), point C (nd=1.90, vd=35.0) and point D (nd=1.835, vd=38.0) in a sequence of A, B, C, D and A as border lines in x-y orthogonal coordinates shown in FIG. 1, in which X-axis is the Abbe number (vd) and Y-axis is the refractive index (nd), the area including the border lines (hereinafter, the above-described area is referred to as a specific area). In particular, an optical glass having the optical constants within the specific area and refractive index (nd) more than 1.85 and less than 1.875, an optical glass having the optical constants within the specific area and Abbe number (vd) less than 39.5 and the like are required.
Cadmium and thorium components have been known for a long time as ingredients providing high refractive index to a glass. However, it is well known that these ingredients cause environmental damage.
A glass containing lead such as PbO is known as a glass having a high refractive index and a low transition temperature (Tg). Since a glass containing lead easily fuses with a metal molding die in precision mold pressing, it is difficult to use a metal molding die repeatedly. Therefore, it is not suitable for a glass of precision mold pressing. Furthermore, for the sake of unfavorable environmental effect of lead, there is a problem that some kinds of environmental measures are required in manufacturing, coolworking such as polishing, disposal and the like of a glass containing lead. There is also a movement to prohibit manufacture, usage and the like of a glass containing lead.
For these reasons an optical glass having high refractive index and low dispersion, low transition temperature, and containing no lead is strongly required.
Since a high refractive index and low dispersion optical glass is useful for a spherical lens as well as an aspherical lens from a viewpoint of optical design, various kinds of the glasses have been suggested for long. For example, Japanese Patent Application Publication (Unexamined) Tokukaisho-52-14607 (hereinafter referred to as Publication 1) discloses a B2O3—SiO2—La2O3—Gd2O3—ZrO2+Ta2O5system optical glass. The glass disclosed in Publication 1 does not fulfill the recent above-described requirement for optical design, because the refractive index is low and the optical constants is not within the above-described specific area. Further it is difficult to perform the precision mold pressing for the sake of its high transition temperature (Tg).
Japanese Patent Application Publication (Unexamined) Tokukaisho-52-155614 (hereinafter referred to as Publication 2) discloses a B2O3—La2O3—Gd2O3—WO3—ZrO2+Ta2O5 system high refractive index and low dispersion optical glass. Though the optical glass disclosed in Publication 2 has the optical constants within the above-described specific area, it is difficult to perform the precision mold pressing for the sake of its high transition temperature (Tg).
Japanese Patent Application Publication (Examined) Tokukousho-54-2646 discloses a SiO2—B2O3—La2O3—Ta2O5+ZnO system high refractive index and low dispersion optical glass. It is difficult that the optical glass disclosed on this publication is applied to precision mold pressing for the sake of its high transition temperature (Tg).
Japanese Patent Application Publication (Examined) Tokukousho-53-4023 discloses a B2O3—La2O3—HfO2 system high refractive index and low dispersion optical glass. It is difficult that the optical glass disclosed on this publication is applied to precision mold pressing for the sake of its high transition temperature (Tg).
Japanese Patent Application Publication (Unexamined) Tokukaisho-54-90218 (hereinafter referred to as Publication 3) discloses a SiO2—B2O3—La2O3—WO3—Ta2O5—Gd2O3—ZrO2 system high refractive index and low dispersion optical glass. It is difficult that the glass practically disclosed in Publication 2 is applied to precision mold pressing for the sake of its high transition temperature (Tg).
Japanese Patent Application Publication (Unexamined) Tokukaisho-52-129716 and Japanese Patent Application Publication (Examined) Tokukousho-54-6042 (hereinafter referred to as Publication 4) discloses a B2O3—La2O3—Y2O3—ZrO2—WO3—Ta2O5+Nb2O5+TiO2 system high refractive index and low dispersion optical glass. It is difficult that both of the glasses disclosed in these publications are applied to precision mold pressing for the sake of their high transition temperature (Tg).
Japanese Patent Application Publication (Unexamined) Tokukaisho-60-46948 discloses a SiO2—B2O3—La2O3—Yb2O3—Nb2O5—Ta2O5 system high refractive index and low dispersion optical glass. It is difficult that the glass practically disclosed in this publication is applied to precision mold pressing for the sake of its high transition temperature (Tg).
Japanese Patent Application Publication (Unexamined) Tokukaisho-60-221338 discloses a B2O3—La2O3—Y2O3-bivalent metal oxide-Li2O system optical glass having low transition temperature (Tg). Since the glass practically disclosed on this publication does not have the optical constants within the above-described specific area, it does not fulfill the above-described recent requirement for optical design.
Japanese Patent Application Publication (Unexamined) Tokukaisho-62-100449 discloses a B2O3—La2O3—ZnO—Li2O—Sb2O3 system optical glass having low transition temperature (Tg). Since this glass disclosed on this publication contains much Sb2O3 as an essential component, in obtaining a glass preform material in hot forming, selective volatilization of Sb2O3 component form the surface layer of fused glass tend to cause the surface striae. The surface striae does not disappear by precision mold pressing, and a glass molding having surface striae does not used for optical elements such as lenses. Therefore, it is not suitable for the glass preform material for precision mold pressing. Further, on precision mold pressing of a glass preform material, Sb2O3 component selectively volatilized from the surface layer of a glass preform material adheres to the molding surface of a metal molding die to cause haze on the molding surface of a metal molding die. If the hazed metal die is used repeatedly, the haze itself is transferred to the surface of a glass molding, or the haze pattern is printed to the surface of a glass molding. Since a glass molding with these problems cannot be used as an optical element such as a lens, it is not suitable for mass production of optical elements such as lenses by precision mold pressing. Furthermore, since the glass practically disclosed on this publication does not have the optical constants within the above-described specific area, it does not fulfill the above-described recent requirement for optical design.
Japanese Patent Application Publication (Unexamined) Tokukaihei-8-217484 (hereinafter referred to as Publication 5) discloses a B2O3—La2O3—Lu2O3 system optical glass. Since the Lu2O3 component contained in this glass as essential component is remarkably expensive, this glass has little practical use because of very high production cost. Among the glasses disclosed on Publication 5, it is difficult that the glasses having the optical constants within the above-described specific area is applied to precision mold pressing for the sake of high transition temperature (Tg).
Japanese Patent Application Publication (Unexamined) Tokukai-2001-348244 (hereinafter referred to as Publication 6) discloses optical glass having glass transition point of not more than 700° C. and optical property of high refractive index and low dispersion. It is difficult that a glass practically disclosed on Publication 6 is applied to precision mold pressing for the sake of high glass transition point (Tg), or transition temperature (Tg).
Japanese Patent Application Publication (Unexamined) Tokukai-2003-267748 (hereinafter referred to as Publication 7) discloses a high refractive index and low dispersion optical glass having low Tg. The glass practically disclosed in Publication 7 having the optical constants within the above-described specific area has a defect of having a difficulty in manufacturing a glass preform material for the sake of its low resistance to devitrification property. Further, there is also a defect that transmission factor in short visible wavelength range is low since a lot of WO3 and TiO2 is contained.
As described above, high refractive index and low dispersion optical glasses according to an earlier development have problems mainly that: though transition temperature is low, its optical constants is not within the above-described specific area strongly required in late years; or that, though its optical constants is within the above-described specific area, the transition temperature (Tg) is high, which cause a difficulty of precision mold pressing.